Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

The description adds useful behavioral detail beyond the annotations by specifying that the file contents are returned base64-encoded in the JSON data field. The annotations already declare readOnlyHint, idempotentHint, and destructiveHint, so the safety profile is covered; the description goes further on output format.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is two concise sentences with no filler. The core action is front-loaded, and the return encoding detail is placed second, making it easy to parse quickly.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a simple, read-only attachment download, the description, schema, and annotations cover the essential behavior. The output schema presumably documents the return shape, so explaining base64 encoding is a helpful addition. It could mention how attachment IDs are discovered, but this is not a blocking gap.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

All three parameters have schema descriptions, so the baseline is 3. The tool description does not add parameter-level meaning beyond what the schema provides, such as how to obtain the IDs or guarantee the relationship between mailboxId, mailId, and attachmentId.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific action, 'Download an attachment,' and identifies the exact resource being acted upon. This clearly distinguishes it from sibling operations like attachments.list, attachments.add, and attachments.delete.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Usage is implied by the verb 'Download': use this when you need attachment content. However, there is no explicit guidance about when not to use it or how it relates to alternatives such as attachments.list, which would help an agent distinguish metadata retrieval from content retrieval.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.